Semiconductor device for power MOS transistor module

ABSTRACT

A first transistor has a first main electrode region which is formed so that these are subdivided into a plurality of first isolated island region. A second transistor has its first main electrode region which are divided into a plurality of second isolated island regions in close proximity to the array of first island regions.

CROSS-REFERENCE TO PRIOR APPLICATION

This application is based upon and claims the benefit of priority fromthe prior Japanese Patent Application No. 2004-115737, filed on Apr. 9,2004, the entire content of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to semiconductor devices, and moreparticularly to a semiconductor device applicable to power metal oxidesemiconductor (MOS) transistor modules for use in protective circuitryof rechargeable batteries.

2. Description of Related Art

Rechargeable or secondary batteries such as lithium-ion batteries or thelike are typically equipped with a protection circuit for protectingsuch secondary batteries against risks of overcharge/over-dischargeand/or over-currents or else. A configuration of this protection circuitis schematically shown in FIG. 10. The illustrative protector circuit100 is generally made up of a serial connection of two transistors QAand QB as connected to a secondary battery unit 200, and a controlintegrated circuit (IC) 300. The control IC 300 monitors a both-endvoltage of the secondary battery 200 and electrical currents flowing intransistors QA and QB. Upon detection of either overcharge/overdischargeor flow of an overcurrent, IC 300 outputs a control signal (gate signal)for causing transistor QA or QB to switch from its conductive state tononconductive state, thereby forcing secondary battery 200 to beelectrically shut off from the load or from the power supply.

As demands grow for miniaturization or down-sizing of handheld wirelesstelephone handsets (mobile phones) or the like, such down-size demandsare becoming more significant also for secondary battery protectioncircuitry. In the light of this trend, an attempt is made to shrink aprotector circuit by arranging two MOS transistors (QA, QB) in aprotector circuit as a power MOS transistor module that is packed ormodularized in a single envelop or housing.

Another requirement for the transistors QA and QB making up such theprotector circuit 100 is that these are low in turn-on (ON) resistivityto thereby enable mobile phones or the like with the secondary battery200 connected thereto to run longer on a single charge-up. In the powerMOS transistor module with the built-in transistors QA-QB also, it isrequired to have low ON resistance as an entirety of the module.

One known power MOS transistor module—i.e., first conventional art—isshown in FIGS. 11 to 13. FIG. 11 is a plan view of a power MOStransistor module in accordance with the first conventional art, andFIGS. 12-13 are cross-sectional diagrams of it as taken along lines A-A′and B-B′ of FIG. 11, respectively.

As shown in FIG. 11, this power MOS transistor module in accordance withthe first conventional art is arranged so that two transistors QA and QBeach having a source region (first main electrode region) and a drainregion (second main electrode region) are formed on a single piece ofmetal substrate (drain frame), wherein the drain regions of respectivetransistors QA and QB are common-coupled together by this metalsubstrate 50. Transistor QA includes a gate electrode 56A and sourceelectrode 57A; transistor QB has a gate electrode 56B and sourceelectrode 57B. The gate electrodes 56A and 56B are applied controlsignals (gate signals) from the above-stated controller IC 300 via gateelectrode wiring lines 53. Source electrodes 57A and 57B are connectedby source electrode wiring lines 54 to external elements or components(such as secondary battery 200, load or the like).

The transistors QA and QB are the so-called trench gate type MOStransistors such as shown in FIGS. 12-13. A trench-gate MOS transistoris the one that is arranged so that the sidewall of a trench gate isused as a channel region, causing a drain current to vertically flowfrom the source region toward the drain region being formed on thesubstrate's back surface. This trench-gate transistor is fabricated in away which follows. Firstly a lightly-doped N (N⁻) type epitaxial layer11 is formed on a semiconductor substrate 10 that was formed as aheavily-doped N (N⁺) type layer. Then, a P-type base layer 12 isselectively formed at a top surface portion of this N⁻-type epitaxiallayer 11. Next, a gate trench 13 is formed from the surface of thisP-type base layer 12 to a depth reaching the N⁻-type epitaxial layer 11.

Then, a gate dielectric film 14 is formed on the inner wall of this gatetrench 13. Further, within this gate trench 13, a gate electrode 15 isformed by bury/embed techniques, which is made of impurity-dopedpolycrystalline silicon or “polysilicon.” Thereafter, an interlayerdielectric film 16 made of silicon oxide is formed at the upper part ofthis gate electrode 15.

Additionally, in surface portions of P-type base layer 12 each of whichis interposed between gate trenches 13, N⁺-type source diffusion layers17 are selectively formed so that one layer 17 is in contact with theside face of gate trench 13. Further at a portion between such N⁺-typesource diffusion layers 17, a P⁺-type diffusion layer 18 is selectivelyformed. In this device structure, when a gate voltage being applied togate trench 13 is controlled, a channel is formed along this N⁺-typesource diffusion layer 17.

This N⁺-type source diffusion layer 17 receives a voltage applied fromthe secondary battery 200 via the source electrode 57A or 57B that isformed at the upper part thereof. Trench gate 13 is given a gate signalfrom the controller IC 300 via gate electrode 56A or 56B in the waystated previously. Gate electrodes 56A and 56B are connected to trenchgates 13 via gate polysilicon wiring lines 58 and extension leads BL(see FIG. 12 or FIG. 13 or else). Gate polysilicon wires 58 are formedabove N-type epitaxial layer 11 with a silicon oxide film 59 sandwichedtherebetween. Note that gate electrode 56A (56B) and source electrode57A (57B) are electrically isolated from each other by an interlayerdielectric film 60 and passivation film 61.

This power MOS transistor module shown in FIGS. 11-13 suffers from alimit in module shrinkage due to the presence of the thickness of metalsubstrate 50.

A power MOS transistor (second conventional art) with module shrinkingcapability is disclosed, for example, in U.S. Pat. No. 6,653,740. Asshown in FIGS. 14-16, this transistor is structured so that the use ofmetal substrate 50 is eliminated, permitting two transistors QA and QBto share a single semiconductor substrate 10 for use as a drain region.An explanation will be given while adding in FIGS. 14-16 the samereference characters to the same parts or components as those of thefirst conventional art. FIG. 14 is a plan view of a power MOS transistormodule in accordance with this second conventional art. FIGS. 15-16 aresectional diagrams of it as taken along lines C-C′ and D-D′ of FIG. 14,respectively.

The transistors QA and QB ate similar in structure to those of the firstconventional art as shown in FIG. 15; however, as shown in FIG. 16, thetransistors QA and QB commonly have or “share” n⁺-type semiconductorsubstrate 10 and n⁻-type epitaxial layer 11. In this structure of thesecond conventional art, a drain current flowing between transistors QAand QB is expected to flow in this semiconductor substrate 10. Thus themetal substrate 50 such as used in the first conventional art becomesunnecessary. This makes it possible to cause the power MOS transistormodule to become less in size than that of the first conventional art.Optionally in this second conventional art, an N⁺-type layer 62 may beprovided at the boundary between transistors QA and QB overlying theN⁻-type epitaxial layer 11.

The second conventional art circuitry is capable of meeting the need forminiaturization of power MOS transistor module as far as its ability toomit the metal substrate 50 is concerned. Unfortunately, the currentflowing between two transistors QA-QB must pass through thesemiconductor substrate 10, resulting in its electrical resistivitybeing greater than that of metal substrate 50 (FIGS. 11-13). Due tothis, the ON resistance of an entirety of the power MOS transistormodule becomes undesirably higher than that of the first conventionalart, which leads to the lack of an ability to fully meet therequirements for reduction of power consumption.

SUMMARY OF THE INVENTION

A semiconductor device in accordance with one aspect of this inventionincludes a pair of first and second transistors which are formedtogether on the same semiconductor substrate. These transistors arearranged to flow a current between a first main electrode region and asecond main electrode region through a channel being formed in a channelregion. Either the first main electrode region or the second mainelectrode region is arranged to be owned in common or “shared” by thepair of first and second transistors. The first transistor is formed sothat it is subdivided into a plurality of first isolated island regions,which are arranged on the semiconductor substrate. The second transistoris formed to be divided into a plurality of second isolated islandregions, which are laid out adjacent to at least part of the firstisolated island regions. At a portion whereat neighboring ones of thefirst isolated island regions face each other, such neighboring firstislands are combined together into a unified first isolated islandregion. In this unified first isolated island region, part of the firsttransistor is integrally formed. At a portion whereat neighboring onesof the second islands face each other, such neighboring second islandsare combined together into a unified second isolated island region. Inthis unified second isolated island region, part of the secondtransistor is integrally formed.

In accordance with another aspect of the invention, a semiconductordevice includes a semiconductor substrate, and a first transistor and asecond transistor formed above the semiconductor substrate. Each of thefirst and second transistors has a first main electrode region formed onone surface side of the substrate, a second main electrode region formedon the other surface side of the substrate, and a gate electrode formedon the one surface side of the substrate for controlling a currentflowable between the first main electrode region and the second mainelectrode region, while letting the second main electrode region beowned in common by the first and second transistors. The first mainelectrode region of the first transistor is formed to be divided into aplurality of first isolated island regions on the one surface side ofthe semiconductor substrate. The first isolated island regions arecommonly connected together to a first electrode wiring line through aplurality of first electrode layers formed at respective upper surfacesof the first isolated island regions. The first main electrode region ofthe second transistor is formed on the one surface side of thesemiconductor substrate and is divided into a plurality of secondisolated island regions in close proximity to the plurality of firstisolated island regions. The plurality of second isolated island regionsare commonly connected together to a second electrode wiring line via aplurality of second electrode layers formed at respective upper surfacesof the second isolated island regions. The gate electrode of the firsttransistor has a first connection region as connected to a firstexternal gate wiring line. The gate electrode of the second transistorhas a second connection region being laid out in a first directiontogether with the first connection region and being connected to asecond external gate wiring line. The plurality of first isolated islandregions and the plurality of second isolated island regions are arrayedin a second direction crossing over the first direction. The pluralityof first isolated island regions include a specified first isolatedisland region which neighbors upon the first connection region and whichhas a first portion aligned in the first direction together with thefirst connection region. The plurality of second isolated island regionsinclude a specified second isolated island region neighboring upon thesecond connection region and having a second portion aligned in thefirst direction together with the second connection region.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a plan view of a power MOS transistor module in accordancewith an embodiment of the present invention.

FIG. 2 shows a plan view of a gate polysilicon wiring line pattern (88A,88B) of the power MOS transistor module in accordance with theembodiment of this invention.

FIG. 3 is a diagram for explanation of a region for fabrication of twotransistors QA and QB.

FIG. 4 shows a cross-sectional view as taken along line E-E′ of FIG. 1.

FIG. 5 shows a sectional view taken along line F-F′ of FIG. 1.

FIG. 6 is a sectional view taken along line G-G′ of FIG. 1.

FIG. 7 shows a modified example of the embodiment of the invention.

FIG. 8 shows another modification of the embodiment of the invention.

FIG. 9 shows still another modification of the embodiment of theinvention.

FIG. 10 schematically shows a configuration of a protection circuit 100for use with a secondary battery unit.

FIG. 11 shows a plan view of a power MOS transistor module in accordancewith the first conventional art.

FIG. 12 shows a cross-sectional view as taken along line A-A′ of FIG.11.

FIG. 13 is a sectional view taken along line B-B′ of FIG. 11.

FIG. 14 shows a plan view of a power MOS transistor module in accordancewith the second conventional art.

FIG. 15 shows a cross-sectional view as taken along line C-C′ of FIG.14.

FIG. 16 is a sectional view taken along line D-D′ of FIG. 14.

DETAILED DESCRIPTION OF THE INVENTION

A first embodiment of this invention will next be explained in detailwith reference to FIGS. 1 through 6 below. FIG. 1 is a plan view of apower MOS transistor module in accordance with the first embodiment.FIG. 2 shows a wiring diagram of a gate polysilicon wiring line pattern(88A, 88B) of this power MOS transistor module. FIG. 3 is a diagram forexplanation of a region in which two transistors QA and QB are to beformed. Additionally, FIGS. 4, 5 and 6 are cross-sectional views takenalong lines E-E′, F-F′ and G-G′ of FIG. 1, respectively. Note here thatthe explanation below is given while adding, in these drawings, the samereference characters to the same parts or components as those in thefirst and second conventional art structures stated in the introductorypart of the description.

As shown in FIGS. 4 and 5, transistors QA and QB of this embodiment alsomay be N-channel trench-gate type MOS transistors each having a trenchgate 13 and N⁺-type source diffusion layer 17 and others in a similarway to that of the first and second conventional art devices statedsupra. These transistors may be fabricated by a similar manufacturingprocess to that stated supra. Note here that this is a mere example, andit is also possible to replace the transistors QA and QB with P-channeltrench gate MOS transistors, by way of example. Other available examplesinclude a device with the transistors QA and QB being replaced byinsulated gate bipolar transistors (IGBTs) or by N-channel/P-channelplanar gate MOS transistors.

As shown in FIG. 6, the transistors QA and QB commonly have or “share” asemiconductor substrate 11 and an N⁻-type epitaxial layer 12. Inaddition, this embodiment does not have the metal substrate 50 used inthe first conventional art and is arranged so that a common drain regionof transistors QA and QB is formed by the semiconductor substrate 11. Inthese points, the power MOS transistor module of this embodiment issimilar to the power MOS transistor module of the second conventionalart.

A principal feature of the power MOS transistor module of thisembodiment is as follows. As shown in FIG. 1, those components thatconstitute the transistor QA, such as a trench gate 13 and an N⁺-typesource diffusion layer 17 and the like, are formed so that these aresubdivided into a plurality of first isolated island regions 71 and 71′.On the other hand, parts making up the transistor QB, such as trenchgate 13 and N⁺-type source diffusion layer 17 and others, are formed sothat they are divided into a plurality of second isolated island regions72 and 72′.

In FIG. 1, the plurality of isolated island regions 71 are designed sothat each has a rectangular shape with its elongate direction beingidentical to the Y axis direction. These island regions 71 are laid outon the semiconductor substrate 10 in such a manner that each is isolatedfrom the others while letting a long side become a neighboring side withrespect to one of the second isolated island regions 72. The pluralityof second isolated island regions 72 are designed so that each has arectangular shape with its elongate side identical to the Y axisdirection. These island regions 72 are laid out among the plurality offirst isolated island regions 71 in such a manner that each is isolatedfrom the others and that a long side becomes a neighboring side withrespect to one of the first isolated island regions 71. To make a longstory short, the first isolated island regions 71 and the secondisolated island regions 72 are alternately arranged to have astripe-shaped layout pattern on the semiconductor substrate 11. Thetransistors QA and QB share a drain region at a surface portion ofsemiconductor substrate 10 which underlies a boundary or “adjacency”line of the both regions.

As shown in FIG. 1, the first isolated island region 71′ and secondisolated island region 72′ are designed to have a letter “L”-like shapewith a combination of a plurality of shape-different rectangles. Thereason of this will be described later.

Also note that as shown in FIG. 1, a source electrode 77A (firstelectrode layer) for supplying a current to N⁺-type source diffusionlayer 17 is formed per first isolated island region 71. Similarly asource electrode 77B (second electrode layer) is formed per secondisolated island region 72.

To this source electrode 77A, a plate-shaped wiring electrode 80A (firstelectrode wiring line)—this is for connection to a first terminal S1 ofthe power MOS transistor module—is connected by a solder bump 81, forexample. Similarly a plate-like wiring electrode 80B (second electrodewiring line), which is coupled to a second terminal S2 of the power MOStransistor module, is connected to the source electrode 77B by a solderbump 81, for example. These plate-like wiring electrodes 80A and 80B areformed so that their elongate direction is identical to a directioncrossing over the adjacency line of the source electrodes 77A and77B—that is, the layout direction of source electrodes 77A and 77B.Wiring electrode 80A is connected only to source electrode 77A; wiringelectrode 80B is coupled only to source electrode 77B. The solder bumps81 may be replaced with gold (Au) bumps. Alternatively, either aluminumbonding wires or Au bond wires may be used for connection between thesource electrode 77A (77B) and wiring electrode 80A (80B).

The transistor QA also has a gate electrode 86A for applying a voltageto trench gate 13. The transistor QB has a gate electrode 86B forapplying a voltage to trench gate 13.

The gate electrode 86A has a solder bump 83 for connection to anexternal circuit (e.g., the controller IC 300 shown in FIG. 10 or thelike) and a plate-like wiring electrode 87A (first external gate wiringline) for connection between the solder bump 83 and the externalcircuit.

Similarly the gate electrode 86B has a solder bump 84 for connection tothe external circuit and a plate-like wiring electrode 87B (secondexternal gate wiring line) for connection between the solder bump 84 andthe external circuit. The wiring electrode 87A and wiring electrode 87Bare drawn out in the opposite direction of that of wiring electrodes 80Aand 80B, with the X-axis direction being as their elongate direction.Additionally, a region 86C (first connection region) in which the wiringelectrode 87A and solder bump 83 are formed and a region 86D (secondconnection region) for fabrication of the wiring electrode 87B andsolder bump 84 are formed with a Y-axis direction as the elongatedirection thereof. These regions oppose each other at their side edgesextending in an X-axis direction.

These gate electrodes 86A and 86B are connected to trench gates 13 viagate polysilicon wiring lines 88A and 88B shown in FIG. 2 and lead wiresBL (see FIG. 5), respectively. As shown in FIGS. 4-6, gate polysiliconwiring lines 88A and 88B are formed above an N⁻-type epitaxial layer 11with a silicon oxide film 89 sandwiched therebetween.

Although a lead wire BL is formed between the gate polysilicon wiringline 88A and trench gate 13 of transistor QA, no such lead wire BL isformed between gate polysilicon wiring line 88A and trench gate 13 oftransistor QB. Similarly, while a lead wire BL is formed between gatepolysilicon wiring line 88B and trench gate 13 of transistor QB, no leadwire BL is formed between gate polysilicon wiring line 88B and trenchgate 13 of transistor QA (see an F-F′ cross-sectional view shown in FIG.5). In this way, only the trench gate 13 of transistor QA is connectedto the gate electrode 86A; only the trench gate 13 of transistor QB iscoupled to gate electrode 86B. With such an arrangement, it is possiblefor the transistors QA and QB to operate in a way independent of eachother.

As shown in FIG. 3, the first isolated island region 71′ has a uniqueshape with an integral combination or “integration” of three firstisolated island regions 71A, 71B and 71C having different rectangularshapes, rather than forming these three regions independently of oneanother. Similarly the second isolated island region 72′ is not formedby two independent second isolated island regions 72A and 72B but formedto have a shape with an integral combination of them. Such shapes areemployed because gate electrodes 86A and 86B are laid out in closeproximity to these regions. Regions 71B and 72A are disposed in the Yaxis direction, while letting their side edges extending together in theX axis direction be as their neighboring sides. In addition, regions 71Cand 72B are arranged in the Y axis direction while being interposedbetween regions 86C and 86D. Region 71C is laid out with its side thatextends in the X axis direction together with region 86C being as aneighboring side. Region 71D is placed with its side that extends in theX axis direction together with region 86D being as a neighboring side.

Here, as shown in FIG. 3, consider the case where the first isolatedisland regions 71A, 71B and 71C and second isolated island regions 72Aand 72B are formed in a narrow region adjacent to the gate electrodes86A and 86B in a way independent of each other. Unlike standard or“regular” isolated island regions 71 and 72, the first isolated islandregion 71B and second isolated island region 72A are laid out with theirX-axis directional sides as the neighboring sides. Similarly, the firstisolated island region 71C and second isolated island region 72B alsoare disposed with their X-axis directional sides as the neighboringsides.

As a result of such difference in direction of the neighboring sides inthis way, the first isolated island regions 71A, 71B and 71C becomeadjacent to one another with their along-the-Y-axis-direction sidesbeing as the neighboring sides. Any one of these regions 71A, 71B and71C is the region that is used for fabrication of the transistor QA.Accordingly, it is possible to enlarge the area of the transistor QA bythe scheme for combining or “uniting” together the three regions 71A,71B and 71C into a single unified region 71′ and then integrally formingtherein a transistor QA as shown in FIG. 3, rather than by the schemefor forming such regions 71A-71C independently of one another. In thesame view point, the scheme for combining together the second isolatedisland regions 72A and 72B into a single united region 72′ and thenintegrally forming therein a transistor QB is more excellent in theability to increase the area of transistor QB. For these reasons, theregions 71′ and 72′ of this embodiment are specifically designed to havethe above-noted shapes.

Note that as shown in a G-G′ cross-sectional view of FIG. 6, the gatepolysilicon wiring lines 88A and 88B are electrically isolated from eachother by an interlayer dielectric film 60 and passivation film 61 plusoxide film 89 and also are electrically insulated from the sourceelectrodes 77A and 77B.

Trench gates 13 and N⁺-type source diffusion layers 17 and others formaking up the transistors QA and QB are formed in the regions surroundedby the gate polysilicon wiring lines 88A and 88B within respectiveisolated island regions 71 and 71′. As apparent from FIGS. 4-6, thetrench gate 13 is formed within the first isolated island region to havea mesh-like planar shape. Note however that this invention is notlimited thereto, and it is also possible to employ a device structurewith a stripe-shaped trench gate 13 being formed in one direction only.

As stated above, in this embodiment, two transistors QA and QB areformed so that these are divided into a plurality of isolated islandregions 71-71′ and 72-72′. Owing to this, the both transistors QA and QBbecome greater in area of counter face therebetween than the first andsecond conventional art structures with each transistor formed in asingle region. Accordingly, as shown in FIG. 6, the both transistors QAand QB share a drain region by means of the semiconductor substrate 11.Even when the silicon forming the semiconductor substrate 11 is high inelectrical resistivity, the resulting area of the counter face becomesgreater than that of the second conventional art. Thus it is possible tolower the turn-on (ON) resistance of the power MOS transistor module asa whole. In addition, with the feature that a source region isisolatively disposed in each region 71, 71′, 72, 72′, it is possible toreadily detect the presence of any defective source region even when theelectrode 81 goes away.

Although the present invention has been disclosed and illustrated withrespect to particular embodiments, this invention should not be limitedthereto and various modifications, additions and replacements areavailable without departing from the scope of the invention. Forexample, the above-noted embodiment is arranged so that each of thefirst and second isolated island regions 71 and 72 has a rectangularshape with the boundary line therebetween being designed as a straightline. However, the invention is not limited only to this arrangement,and other schemes are available. An example is shown in FIG. 7, whereinthe boundary line between first and second isolated island regions 71-72is arranged to have a wavy line shape. With this arrangement, it ispossible to further increase the area of the counter face of twotransistors QA and QB. This makes it possible to further lower the ONresistance. Another example is that the boundary line is designed as aconcave-and convex curve as shown in FIG. 8. Alternatively the boundaryis designable as an arc-like curve as shown in FIG. 9. Using any one ofthese approaches makes it possible to enlarge the area of the counterface of both transistors QA and QB. Also note that the shape of anisolated island region also is not limited to the rectangle and may bedesigned into other shapes including, but not limited to, ellipses andpolygons. It is also possible to form on the back surface ofsemiconductor substrate 10 a metallic film with its thickness as thin aspossible to an extent that it hardly becomes a substrate, thereby makingit possible to further lower the resistance value of the common drainregion.

1. A semiconductor device comprising: a semiconductor substrate; and afirst transistor and a second transistor formed on said semiconductorsubstrate, each of the first and second transistors having a first mainelectrode region formed on one surface side of said substrate, a secondmain electrode region formed on the other surface side of saidsubstrate, and a gate electrode for control of a current flowablebetween said first main electrode region and said second main electroderegion, while letting said second main electrode region be owned incommon by said first and second transistors, wherein said first mainelectrode region of said first transistor is formed to be divided into aplurality of first isolated island regions on the one surface side ofsaid semiconductor substrate, said first main electrode region of saidsecond transistor is formed on the one surface side of saidsemiconductor substrate and is divided into a plurality of secondisolated island regions as laid out adjacent to at least part of saidplurality of first isolated island regions, at a portion whereatneighboring ones of said first isolated island regions face each other,the neighboring first isolated island regions are combined together intoa unified first isolated island region, and in this unified firstisolated island region said first main electrode region of said firsttransistor is integrally formed, at a portion whereat neighboring onesof said second isolated island regions face each other, the neighboringsecond isolated island regions are combined together into a unifiedsecond isolated island region, and in this united second isolated islandregion said second main electrode region is integrally formed, saidfirst isolated island regions and said second isolated island regionsare organized into at least first groups being alternately placed whileletting a contour extending in a first direction be an adjacent portionand second groups being alternately placed while letting a contourextending in a second direction other than said first direction be anadjacent portion, one of the first isolated island regions of said firstgroups and its neighboring one of the first isolated island regions ofsaid second groups are combined together into said unified firstisolated island region, and one of the second isolated island regions ofsaid first groups and its neighboring one of the second isolated islandregions of said second groups are combined together into said unifiedsecond isolated island region.
 2. The semiconductor device according toclaim 1, wherein said gate electrode of said first transistor has afirst connection region for connection to an external wiring line, saidgate electrode of said second transistor has a second connection regionfor connection to an external wiring line, and the second isolatedisland regions of said second groups have an array of portions as laidout in said first direction together with said first connection regionand said second connection region.
 3. The semiconductor device accordingto claim 2, wherein the second isolated island region of said secondgroups is placed between said first connection region and said secondconnection region.
 4. The semiconductor device according to claim 1,wherein a boundary line between said first isolated island regions andsaid second isolated island regions is a straight line in shape.
 5. Thesemiconductor device according to claim 1, wherein a boundary linebetween said first isolated island regions and said second isolatedisland regions has a shape like a curved line with an alternate layoutof protuberances and depressions.
 6. The semiconductor device accordingto claim 1, wherein a boundary line between said first isolated islandregions and said second isolated island regions is a wavy line in shape.7. A semiconductor device comprising: a semiconductor substrate; and afirst transistor and a second transistor formed above said semiconductorsubstrate, each of the first and second transistors having a first mainelectrode region formed on one surface side of said substrate, a secondmain electrode region formed on the other surface side of saidsubstrate, and a gate electrode formed on the one surface side of saidsubstrate for controlling a current flowable between said first mainelectrode region and said second main electrode region, while lettingsaid second main electrode region be owned in common by said first andsecond transistors, wherein said first main electrode region of saidfirst transistor is formed to be divided into a plurality of firstisolated island regions on the one surface side of said semiconductorsubstrate, said first isolated island regions being commonly connectedtogether to a first electrode wiring line through a plurality of firstelectrode layers formed at respective upper surfaces of said firstisolated island regions, said first main electrode region of said secondtransistor is formed on the one surface side of said semiconductorsubstrate and is divided into a plurality of second isolated islandregions in close proximity to said plurality of first isolated islandregions, said plurality of second isolated island regions being commonlyconnected together to a second electrode wiring line via a plurality ofsecond electrode layers formed at respective upper surfaces of saidsecond isolated island regions, said gate electrode of said firsttransistor has a first connection region as connected to a firstexternal gate wiring line, said gate electrode of said second transistorhas a second connection region being laid out in a first directiontogether with said first connection region and being connected to asecond external gate wiring line, said plurality of first isolatedisland regions and said plurality of second isolated island regions arearrayed in a second direction crossing said first direction, saidplurality of first isolated island regions including a specified firstisolated island region neighboring upon said first connection region andhaving a first portion aligned in said first direction together withsaid first connection region, and said plurality of second isolatedisland regions including a specified second isolated island regionneighboring upon said second connection region and having a secondportion aligned in said first direction together with said secondconnection region.
 8. The semiconductor device according to claim 7,wherein said specified first isolated island region has a third portionextending in the first direction together with a remaining portion ofsaid specified second isolated island region other than said secondportion thereof.
 9. The semiconductor device according to claim 8,wherein said first portion and said second portion are placed betweensaid first connection region and said second connection region.
 10. Thesemiconductor device according to claim 8, wherein said first electrodewiring line and said second electrode wiring line are plate-likeelectrodes.
 11. The semiconductor device according to claim 8, whereinsaid first electrode wiring line and said second electrode wiring lineare plate-like electrodes with their elongate direction identical to adirection along which said plurality of first main electrodes and saidplurality of second main electrodes are queued.
 12. The semiconductordevice according to claim 11, wherein said first external gate wiringline and said second external gate wiring line are plate-likeelectrodes.
 13. The semiconductor device according to claim 11, whereinsaid first external gate wiring line and said second external gatewiring line are plate-like electrodes with their elongate directionidentical to a direction along which said plurality of first mainelectrodes and said plurality of second main electrodes are lined up.14. The semiconductor device according to claim 13, wherein said firstexternal gate wiring line and said second external gate wiring line areextended in an opposite direction to said first electrode wiring lineand said second electrode wiring line.
 15. The semiconductor deviceaccording to claim 7, wherein a boundary line between said firstisolated island regions and said second isolated island regions is astraight line in shape.
 16. The semiconductor device according to claim7, wherein a boundary line between said first isolated island regionsand said second isolated island regions has a shape like a curved linewith an alternate layout of protuberances and depressions.
 17. Thesemiconductor device according to claim 7, wherein a boundary linebetween said first isolated island regions and said second isolatedisland regions is a wavy line in shape.
 18. The semiconductor deviceaccording to claim 7, wherein said plurality of first electrode layersand said plurality of second electrode layers are alternately placed inthe second direction.
 19. The semiconductor device according to claim 7,wherein said first electrode layers are connected through bumps to saidfirst electrode wiring line whereas said second electrode layers areconnected via bumps to said second electrode wiring line.
 20. Thesemiconductor device according to claim 7, wherein said first electrodelayers are connected by bonding to said first electrode wiring linewhereas said second electrode layers are connected by bonding to saidsecond electrode wiring line.